By exposing Hep-2 cells in tissue culture and erythrocytes to chemicals in nontoxic concentrations and then to long ultraviolet and visible light, a photosensitizing cytotoxicity was demonstrated with 17 different compounds representing ten different classes of chemicals. Some compounds demonstrated a prompt and irreversible interaction with the cells while others required a delay of 60 to 90 minutes for the interaction to occur. The bond between cell and drug tended to become more firm with longer incubation until it was irreversible by washing of the cells. No effect on acid phosphatase or succinic dehydrogenase staining was detected.